Method of operating a coulometric device

ABSTRACT

A method for operating small coulometric devices having liquid metal electrodes is disclosed. The maximum operating current which such devices are capable of absorbing may be increased ten fold and more by superposing on the conventional direct operating current an alternating pacifying current.

Umted States Patent [151 3,686,566 Beusman [451 Aug. 22, 1972 54] METHOD OF OPERATING A [56] References Cited 72 ICOULOMETRIC DEVICE I UNITED STATES PATENTS I 1 3"? W 3,462,684 8/1969 Beusman ..324/1s2 x 3,255,413 6/1966 Marwell et a1 ..324/94 [73] Assignee: Curtis Instruments, Inc., New York, NY. Primary'Examiner-Alfred E. Smith Att0meyPennie, Edmonds, Morton, Taylor and [22] Filed: April 20, 1970 Adams 21 Appl. No.: 30,159 [57] ABSTRACT A method for operating small coulometric devices [52] US. Cl ..324/94 having liquid metal electrodes is disclosed The [5 "G011. imum operating current such devices are capa. [58] Field of Search ..324/94, 182, 761 N ble of absorbing may be increased ten fold and more by superposing on the conventional direct operating current an alternating pacifying current.

4 Claims, 1 Drawing Figure '2 1s ,1 3 I7 2 D C I R A C Source I Source METHOD OF OPERATING A COULOMETRIC DEVICE BACKGROUND OF THE INVENTION The basic miniaturized coulometric device disclosed in US. Pat. No. 3,045,178 to Corrsin has proved to have many practical applications. From the beginning, however such devices have had limited utility in high rate integrations, i.e., currents through the device which are sufficiently great to displace the electrolyte gap between the liquid metal columns along substantially the full length of the bore in times as short as 1 minute. Practical considerations relating to the geometry of these microcoulometers and the solubility limits of the various salts used in the electrolytes have inherently limited the rate at which the electrolyte gap may be displaced along the bore. These limitations also greatly extend the time and cost of setting each new device to zero during manufacture and they make it impractical for the user to reset any device which has been run for any substantial length of time. Furthermore, these limitations impose severe restrictions on the designers freedom to create a wide range of devices having different scale lengths and full scale operating times.

While seeking means to overcome these limitations I discovered a new method of operating a liquid metal coulometric device that permits a device of a given size and construction to be operated at very much greater current densities than was previously pemiissible or practical. The new method is useful not only in operating the high rate devices referred to above, but in operating the more conventional devices as well.

SUMIVIARY OF THE INVENTION The invention relates to a method of operating a miniaturized coulometric device. Generally, such a device comprises a non-conductive body having a bore therein. Within the bore there are two liquid metal columns, each colurrm filling the cross-section of the bore and extending from one end of the bore toward the other end such that a gap exists between the inmost ends of the columns. The gap is filled with a liquid electrolyte material that is in conductive contact with the inmost ends of the columns. Conductive means are provided for conductively connecting the two metal columns in circuit with sources of electric current.

As is well known, coulometric devices of this kind must be operated with direct current such that one column is the anode and the other is the cathode. Liquid metal is transported by electrolytic action from the anodic column to the cathodic column with the result that the anode grows shorter and the cathode becomes longer. The displacement of the electrolyte gap along the bore provides a discernible analog of the quantity of electric charge passed through the device. As is equally well known, any attempt to energize such a coulometric device with alternating current would result in no displacement of the electrolyte gap and, therefore, no detectable measure of the electric charge, for the electrolytic functions of the columns will follow the instantaneous changes in polarity of the alternating current with no net transport of metal across the gap. It has now been found, however, that there is a very substantial improvement to be realized by operating coulometric devices with both direct and alternating current applied to the electrode columns.

In accordance with the invention the new method comprises the step of energizing the device by application of direct operating current through the two metal columns and the electrolyte together with the step of pacifying the electrolytic reactions due to said operating current by superimposing an alternating current on said operating current.

Operation of miniaturized coulometric devices in accordance with the new method permits tenfold and greater increases in electrolytic current densities with consequent increases in the rate of electromechanical transport of liquid metal through the electrolyte from one metal column to the other. The rate of displacement of the gap between the ends of the columns may be commensurately increased as may the sensitivity of the device.

A further benefit of the new method is a marked reduction in the agitation of the metal-electrolyte interfaces, especially at high current densities. The reduction in stirring and convective density gradients at the metal-electrolyte interfaces also facilitates detection of the gap position by photoelectric and capacitive means.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE shows in greatly enlarged cross-section a typical rnicrocoulometer in combination with a schematic representation of electrical current sources necessary to operation of the coulometer in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the drawing a typical rnicrocoulometer l to be operated in accordance with the invention comprises a tubular body 2 of transparent non-conductive material,

such as glass or resin. Within the bore of tube 2 are two columns 3 and 4 of a liquid metal, such as mercury. Each of these columns extends from an end of the bore toward the other column. Depending on the particular use for which the coulometer is designed, the diameter of the bore within the tubular body 2 may be constant or variable along the length of the bore, but the diameter is typically less than about 3 millimeters so that capillary action of the liquid metal columns prevails.

Between the inmost ends 5 and 6 of the columns 3 and 4, respectively, there is a gap or space in the bore which is filled with any suitable electrolyte 7 which may be an aqueous solution of a mercury salt, such as mercuric iodide. The electrolyte is in conductive contact with the ends 5 and 6 of the columns. The ends of the bore in the body 2 are closed by seals 8 and 9 which may be formed of an epoxy resin.

Means are provided for making electrical contact with the liquid metal columns. These may be fine contact wires 10 and 11 which are formed of some conductive material such as platinum, that is non-reactive with mercury. Each of the wires extends through one of the seals 8 and 9 at the opposite ends of the bore and into good conductive contact with the liquid metal column at that end.

The device described thus far is now well known and available in several different forms on the commercial market. Normally, a device of this kind is energized by means of the contacts 10 and 11 being connected in circuit with a source of direct operating current. The electrical current through the device causes an electrolytic action to occur which results, by well known means, in the transport of metal across the electrolyte from one column to the other. As this transport progresses, the one column becomes shorter and the other becomes longer with a consequent displacement of the volume of electrolyte along the bore of the tubular body 2. In accordance with Faradays law the mass of liquid metal transported across the electrolyte gap is directly proportional to the quantity of electrical charge flowing in the energizing circuit. Thus, if the bore of the tube and the operating current are constant, the electrolyte gap will be displaced along the tube at a rate determined by the magnitude of the current. The device may be calibrated so that the total displacement of the electrolyte will be a measure of the time the device is energized by the operating current.

In accordance with the invention the coulometer l is energized by any suitable direct current source 12, one terminal of which source is electrically connected to the wire contact by conductors l3 and 14. The other terminal of source 12 is electrically connected to wire contact 11 as indiacted by the ground symbols 15. The circuit thus established through the coulometer provides the primary operating current.

In accordance with the invention there is also applied to the coulometer an alternating current for pacifying the effects of the electrolytic actions at the interfaces between the liquid mercury columns and the electrolyte. The source of current for this purpose is indicated at 16, and, as shown, one terminal is connected to electrode 10 through a dc. blocking capacitor 17 and conductor 14, while the other terminal of the alternating current source is connected to electrode 11 through ground 15.

Microcoulometers are typically low resistance devices. They may be connected in circuit with a large variety of electronic and electrical devices which may serve as the current sources for the coulometer. As will be understood by those skilled in the art, some such devices may be characterized as low impedance voltage generators, others may be extremely high, or infinite, impedance current generators, and others may be of some intermediate impedance. Those skilled in the electrical and electronic arts will understand that the coulometer may be satisfactorily coupled to both the alternating current and direct current sources by various known means, the selection of which will depend on the relative impedances of the coulometer and the current source and upon the voltage of the source. Thus, the capacitive coupling of the coulometer to the alternating current source as shown in the FIGURE is only illustrative of one of many possible modes well known to those skilled in the art. When the source is of very high impedance or high voltage the coulometer may be coupled to the source through a series resistance of suitable value.

As an illustration of the improvement attributable to the new method, it has been found that a coulometer having mercury columns in a 0.012 inch bore, but with a highly saturated electrolyte solution, could be operated with direct currents of not more than 0.5 milliamperes. However, when operated in accordance with the invention by application of a 60 cycle per second pacifying current of approximately 1 milliampcre, the dc. operating current through the device could be increased by more than tenfold, i.e., in excess of 5 milliamperes, with a consequent increase in the rate of displacement of the electrolyte gap and without the disabling effects of the electrolytic reactions which would otherwise occur at such high current densities.

It has been found that the most effective frequency of the alternating pacifying current is less than about 2,000 cycles per second. It has also been found that an advantageous effect of the method in accordance with the invention occurs when the amplitude of the pacifying current is such as to effect short term functional inversions of the two electrode surfaces 5 and 6; that is, the relative magnitudes of the direct operating current and the alternating pacifying current are such that the cathodic interface becomes anodic for a short portion of the alternating current cycle and the anode becomes cathodic.

Microscopic examination of the interfaces of a device operated in accordance with the invention reveals a substantial reduction in the agitation of the surfaces of the ends of the mercury columns as well as a substantial reduction in the observable convection currents within the electrolyte, even though the quantity of mercury actually transported across the electrolyte gap is greatly increased. Additionally, it has been found that the disabling effects of anodic precipitation of electrolyte salts are greatly reduced as are the disruptive effects of increases in surface tension at the interface of the normally cathodic liquid metal column. Electronic noise generated by such devices is also substantially reduced when operated by the new method.

The new method of operating rnicrocoulometers is applicable to a large variety of particular devices. The scope of the invention is defined by the following claims.

I claim:

1. The method of operating a coulometric device including a non-conductive body having a bore therein, two columns of liquid metal within said bore, each column extending from an end of said bore toward the other column such that a gap not filled by column metal exists between the endmost ends of the columns and a liquid electrolyte disposed within said gap and being in conductive contact with the endmost ends of said columns, said device being characterized by a maximum practical operating current in the absence of alternating current, which method comprises the steps of:

energizing the coulometer with a direct operating current exceeding said maximum practical operating current in the absence of alternating current, the polarity of said operating current determining the normal coulometric function of said liquid metal columns; and

superimposing on said operating current an alter-nating current having a frequency of less than about 2,000 cycles per second for pacifying the efiects of electrolytic action due to said operating current.

2. The method of operating a coulometric device according to claim 5 wherein the frequency of the pacifying alternating current is about 60 cycles per second.

3. The method of operating a coulometric device according to claim 1 and in which the amplitude of the pacifying alternating current is sufficient to cause an inversion of the normal coulometric functions of the liquid metal columns during a portion of each cycle of the pacifying current.

4. The method of operating a coulometric device according to claim 1 and in which the frequency of the 5 pacifying alternating current is less than about 1,000 cycles per second.

mg I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIN patent 3 ,686 ,566 a d August 22, 1972 m- Curtis C. Beusman It is eertified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

. r- Column 4 line 64 (line 2 of claim 2) delete ,"5" '1 and substitute 1 Signed and sealed this 9th day of January 1973.

(SEAL) Attest:

- ROBERT GGTTSCI'TALK Commissioner of Patents EDWARD M.FL.E'ICI-IER,JR

[\ttesting Officer 

1. The method of operating a coulometric device including a nonconductive body having a bore therein, two columns of liquid metal within said bore, each column extending from an end of said bore toward the other column such that a gap not filled by column metal exists between the endmost ends of the columns and a liquid electrolyte disposed within said gap and being in conductive contact with the endmost ends of said columns, said device being characterized by a maximum practical operating current in the absence of alternating current, which method comprises the steps of: energizing the coulometer with a direct operating current exceeding said maximum practical operating current in the absence of alternating current, the polarity of said operating current determining the normal coulometric function of said liquid metal columns; and superimposing on said operating current an alternating current having a frequency of less than about 2,000 cycles per second for pacifying the effects of electrolytic action due to said operating current.
 2. The method of operating a coulometric device according to claim 5 wherein the frequency of the pacifying alternating current is about 60 cycles per second.
 3. The method of operating a coulometric device according to claim 1 and in which the amplitude of the pacifying alternating current is sufficient to cause an inversion of the normal coulometric functions of the liquid metal columns during a portion of each cycle of the pacifying current.
 4. The method of operating a coulometric device according to claim 1 and in which the frequency of the pacifying alternating current is less than about 1,000 cycles per second. 